In a scanning electron microscope (“SEM”), a primary beam of electrons is scanned onto a region of a sample that is to be investigated. The energy released in the impact of the electrons with the sample causes the liberation of other charged particles in the sample. The quantity and energy of these secondary particles provide information on the nature, structure and composition of the sample. The term “secondary electron” as used herein includes backscattered primary electrons, as well as electrons originating from the sample. To detect secondary particles, a SEM is often provided with one or more secondary electron detectors. The term “sample” is traditionally used to indicate any work piece being processed or observed in a charged particle beam system and the term as used herein includes any work piece and is not limited to a sample that is being used as a representative of a larger population.
In a conventional SEM, the sample is maintained in a high vacuum to prevent scattering of the primary electron beam by gas molecules and to permit collection of the secondary electrons. However, wet samples such as biological specimens are not suitable for observation in a high vacuum. Such samples experience evaporation of their fluid content in the vacuum before an accurate image can be obtained, and the evaporated gas interferes with the primary electron beam. Objects that outgas, that is, solids that lose gas at high vacuum, also require special consideration.
Electron microscopes that operate with the sample under a relatively high pressure are described, for example, in U.S. Pat. No. 4,785,182 to Mancuso et al., entitled “Secondary Electron Detector for Use in a Gaseous Atmosphere.” Such devices are better known as High Pressure Scanning Electron Microscopes (HPSEM) or Environmental Scanning Electron Microscopes. An example is the Quanta 600 ESEM® high pressure SEM from FEI Company.
In an HPSEM, the sample that is to be investigated is placed in an atmosphere of a gas having a pressure typically between 0.1 Torr (0.13 mbar) and 50 Torr (65 mbar), and more typically between 1 Torr (1.3 mbar) and 10 Torr (13 mbar) whereas in a conventional SEM the sample is located in vacuum of substantially lower pressure, typically less than 10−5 Torr (1.3×10−5 mbar). The advantage of an HPSEM as compared to a conventional SEM is that the HPSEM offers the possibility of forming electron-optical images of moist samples, such as biological samples, and other samples which, under the high vacuum conditions in a conventional SEM, would be difficult to image. An HPSEM provides the possibility of maintaining the sample in its natural state; the sample is not subjected to the disadvantageous requirements of drying, freezing or vacuum coating, which are normally necessary in studies using conventional SEMs. Another advantage of an HPSEM is that the ionized imaging gas facilitates neutralization of electrical charges that tend to build up on insulating samples, such as plastics, ceramics or glasses.
In an HPSEM, secondary electrons are typically detected using a process known as “gas amplification,” in which the secondary charged particles are accelerated by an electric field and collide with gas molecules in an imaging gas to create additional charged particles, which in turn collide with other gas molecules to produce still additional charged particles. This cascade continues until a greatly increased number of charged particles are detected as an electrical current at a detector electrode. In some embodiments, each secondary electron from the sample surface generates, for example, more than 20, more than 100, or more than 1,000 additional electrons, depending upon the gas pressure and the electrode configuration.
HPSEM limit the region of high gas pressure to a sample chamber by using a pressure-limiting aperture (PLA) to maintain a high vacuum in the focusing column. Gas molecules scatter the primary electron beam, and so the pressure limiting aperture is positioned to minimize the distance that the electron beam travels in the high pressure region to reduce interference with the primary beam, while providing a sufficient travel distance for adequate gas amplification of the secondary electron signal.
An HPSEM as described in U.S. Pat. No. 4,785,182 comprises a vacuum envelope having a pressure limiting aperture, an electron beam source located within the vacuum envelope and capable of emitting electrons, focusing lens located within the vacuum envelope and capable of directing an electron beam emitted by the electron beam source through the pressure limiting aperture, beam deflectors located within the vacuum envelope and capable of scanning the electron beam, and a sample chamber including a sample platform disposed outside the high vacuum envelope and capable of maintaining a sample enveloped in a gas at a desired pressure.
While an HPSEM can observe moist biological sample, problems still exist with such observations. For example, when hydrated materials are observed at room or body temperature, water tends to condense on all surfaces within the sample chamber. Such condensation can interfere with the operation of HPSEM, as well as cause corrosion and contamination.
Charged particle beams, such as electron beams or ion beams, can also be used to induce a chemical reaction to etch a sample or to deposit material onto a sample. Such processes are described, for example, in U.S. Pat. No. 6,753,538 to Mucil et al. for “Electron Beam Processing.” The process of a charged particle beam interacting with a process gas in the presence of a substrate to produce a chemical reaction is referred to as “beam chemistry.” The term “processing” as used herein includes both processing that alters the sample surface, such as etching and deposition, as well as imaging. The term “processing gas” is used to include a gas that is used for imaging or a gas that is used together with the charged particle beam to alter the sample. The term “imaging gas” is used to include a gas that is used for imaging. The classes of gasses are not mutually exclusive, and some gases may be used for both altering the sample and for forming an image. For example, water vapor can be used to etch a sample that includes carbon and can be used to form an image of samples that include other materials.
Conventional HPSEMs are not well adapted for efficient beam chemistry. One problem with using a HPSEM system for beam chemistry is the considerable time required to introduce and evacuate gases from the sample chamber. The sample chamber in a conventional HPSEM includes a gas inlet through which a gas is introduced through a leak valve. The gas then migrates throughout the sample chamber. Some of the gas molecules escape through the PLA into the column, where they are removed by a vacuum pump that maintains the column at a low pressure. The inlet leak valve is adjusted so that a desired equilibrium pressure is achieved, with the gas escaping through the PLA into the column just matching the gas introduced through the leak valve. The HPSEM typically begins processing a sample only after equilibrium is achieved. It takes a considerable amount of time for the gas to reach an equilibrium pressure, particularly if the vapor pressure of the beam chemistry precursor is similar to the desired operating pressure of the sample chamber. For a large sample chamber volume of 30 liters, such as that used in a typical dual-beam system, it can take up to 30 minutes for the partial pressure of the process gas to reach equilibrium.
This problem is compounded when a process entails injecting multiple process gases into the sample chamber. Typically, there is a pressure gauge downstream of the needle valve on the chamber side. The pressure gauge measures the total pressure in the sample chamber and is incapable of separately measuring the partial pressures of multiple process gases in a mixture. Thus, it is difficult to know when the desired partial pressure of each of the gases has been achieved.
When performing beam chemistry processing in a conventional SEM, FIB, or dual beam system, the system operator will typically obtain a charged particle beam image of the sample to navigate to an area that is to be processed by etching or depositing material. After performing the beam processing operation, the operator will typically obtain another charged particle beam image of the sample to evaluate the results of the process. Because different gases are typically used to process and image in an HPSEM, the sequence of image, process, and image would require multiple changes of the gas in the chamber. If some process gas remains in the chamber during imaging, the sample may be unintentionally modified by the beam during the imaging operation. Because of the considerable time required to fully evacuate one gas and then to reach equilibrium pressure with another gas, such multiple step operations are not practical in an HPSEM. The time is further increased in some cases because the molecules of some gases used in beam chemistry tend to have very long adsorption times on the vacuum chamber walls, and take longer to fully evaporate.
GB2186737 to Shah describes a specimen chamber for use in a scanning electron beam instrument. A sample sits atop a specimen support including a moisture absorbing pad. An inlet duct connected to a source of ambient gas brings moisture from the pad into the chamber to keep the sample from drying out. An electrode positioned close to the sample allows charged carriers of either sign to be removed from the chamber to improve contrast of an image derived from current detected at the specimen support.
Another reason why HPSEMs are not generally used for beam chemistry is that corrosive process gases can degrade the HPSEM components. For example, certain process gases associated with beam chemistry can react spontaneously with plastic tubing and are very dangerous to human health. A gas like XeF2 can make plastic gas tubing brittle and eventually cause leaks of dangerous gases into the surrounding environment.